You can feed them, my longspine urchin enjoys flake food and pellets (just need to shoot the food using a turkey baster into the spines - it finds the food - it's kindof freaky watching an urchin catch flake food, who'da thunk they could do that??
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But, no way would I have mine in a 10g. When I got mine it was small, like 2" across tip-to-tip, body the size of a dime... but it was body the size of a quarter and 6" across within a couple weeks ... and body the size of a toonie a couple weeks later ... I have no idea it's fully extended length nowadays but the body is the size of a raquetball. Anyhow I was surprised how quickly it grew .. so I don't think a 10g would work well I'm afraid.
You might want to look into making a surrogate urchin for those banggai fry. Probably could get away with something as simple as some underwater epoxy wadded into a ball, and a bunch of swizzle sticks or narrow straws jammed into it...